Differential growth rates, shelter competition, food competition, and aggression displays between O. rusticus and O. virilis.

Abstract

O. rusticus and O. virilis are two species of crayfish that live in the Great Lakes region. O. rusticus is a recent invader in this region, and its appearance has been correlated with a decline in numbers of native O. virilis. The objective of this experiment was to compare growth rates, shelter competition, feeding competition, and aggressive behavior between the two species. Prior research has indicated that O. rusticus grows faster and is aggressive towards O. virilis. The results of the experiment found no significant difference between growth rates, shelter competition, feeding competition, and aggressive behavior for O. rusticus and O. virilis. The findings were probably inaccurate due to procedural and equipment problems. Another anti-predatory tactic used by prey species is schooling. Schooling provides defense and protection for individuals within the school. By definition, a polarized school is comprised of similar size individuals who synchronize their speed and movement. Two working hypotheses were created from these observations. First, that N. stramineus will exhibit reactions to both visual and olfactory cues given from a predator, and vision will be the most significant cue of the two. Tests introduced N. stramineus to visual and olfactory cues independently of each other to determine the importance of each. Second, that schooling patterns show variation with size of N. stramineus individuals in the school so that oddly sized individuals will be situated in the center of the school. Individuals of varying size were added to schools of uniform size to determine distributions among them. A significant difference was found between visual and two different olfactory cues. N. stramineus reacted strongest to the visual cues. The two different olfactory cues were not significantly different. It was also found that large N. stramineus individuals tend to situate the center of the school, whereas small individuals were most often found on the outside of the school.